In the prior art, it is well known that some zoom lenses are adapted to simply displace the leading group of lens pieces closest to a subject being photographed for the focusing, which may be referred to as a ‘front lens focusing system’. With such a front lens focusing system, switching the focusing mode to ‘Automatic’ forces the large heavy lens pieces to move instantaneously, resulting in an unsatisfactory rapidity.
Additional types of the focusing system, which are known as ‘inner-focusing’ and ‘rear-focusing,’ have been developed in the art which enable rapid focusing by virtue of the reduced dimensions of the focusing lens groups.
With such inner-focusing and rear-focusing, however, their rapid focusing ability is a tradeoff of more largely varied aberration, and as a result of the focusing, it becomes hard to compensate well for so much variation in the aberration.
In a specific type of the zoom lens, which is for telephoto shooting with a field angle 2ω of less than 40 degrees at the wide-angle end, a positive lens piece(s) in the first lens group has its convex surface faced toward the image plane while a negative lens piece(s) closest to the subject being photographed in the second lens group has it objective surface flattened or convex in shape and faced toward the subject being photographed, so as to reduce variations in spherical aberration at the telephoto end, thereby facilitating the compensation, which is a tradeoff of considerable difficulties in adjusting and controlling the varied curvature of the image plane at the wide-angle end.
For instance, one typical inner-focusing zoom lens already disclosed is of five groups of lens pieces, including their respective optical properties, i.e., of positive, negative, positive, positive, and negative refractivities, respectively (see Patent Document 1 listed below).
Another zoom lens coping with the aforementioned disadvantage in the art is of six groups of lens pieces where only the fifth succeeding to the first or the leading group of them are displaced for the focusing (see Patent Document 2 listed below).
Still another typical zoom lens in the prior art is of six groups of lens pieces, namely, the ones respectively having positive, negative, positive, negative, positive, and negative refractivities, and the sixth serving as the primary focusing lens along with the fourth behave as floating lens during the focusing for a shift from the long distance zooming to the short distance zooming (see Patent Document 3 listed below).
Patent Document 1    Japanese Patent Publication of Unexamined Application No. 2005-292338
Patent Document 2    Japanese Patent Publication of Unexamined Application No. H10-133107
Patent Document 3    Japanese Patent Publication of Unexamined Application No. 2000-47107
The zoom lens as disclosed in Patent Document 1 has a problem of significant variation in curved image plane at the wide-angle end due to a negative lens piece closest to the subject being photographed in the second lens group that has its surface closer to the subject shaped in concave.
In the zoom lens as disclosed in Patent Document 2, the first or foremost of the lens groups under-correct the varied spherical aberration during the focusing for the shift from the long distance zooming to the short distance zooming, but instead, the fifth lens group permits the variations in spherical aberration to be overdone and prevalent by means of connecting a single negative lens piece and another lens piece into a composite lens with its junction serving to diverge incident beams, so as to countervail the variations in the spherical aberration in the lens optics, as a whole. Configured in this manner, however, the zoom lens fails to sufficiently adjust the varied spherical aberration as desired in the comprehensive lens optics, especially, at the telephoto end, due to the insufficient diverging effects at the junction of the composite lenses.
Patent Document 2 also teaches the fifth and the fourth of the lens groups behave in the floating manner for focusing. This is especially for adjusting and controlling both the curved image plane at the wide-angle end and the varied spherical aberration at the telephoto end. In contrast with the one that carries out the focusing simply relying on the inner-focusing system, however, this prior art embodiment becomes more complicated in structure because of additional coupling components to a lens barrel to cope with the floating.
According to the disclosure of Patent Document 3, the first to the fourth of the lens groups under-correct the varied spherical aberration, and instead, the fifth turns the same overdone and prevalent during the focusing, which permit the variations in spherical aberration to be reduced in the lens optics, as a whole. However, the negative lens of the composite lens effects insufficient to diverge the incident beam thereon to satisfactorily compensate for the varied spherical aberration at the telephoto end. Moreover, the teachings about the focusing where the fourth and the six of the lens groups behave in a floating manner bring about another problem of the more complicated structure due to the coupling components added to the lens barrel to conduct the floating.
During focusing for the shift from long distance zooming to short distance zooming, in general, the zoom lens is prone to under-correct the variations in spherical aberration unless it is specifically modified to address the desired reduction in displacement of the focusing lens groups or modified in some other ways, and this tendency is conspicuous especially at the telephoto end. As to the zoom lenses designed specifically for telephoto shooting where a focal length is as long as 400 mm, degradation in optical performance is considerable during the focusing for the short distance zooming.
The present invention is made to overcome the aforementioned disadvantages in the prior art that zoom lenses employing a focusing system such as inner-focusing, rear-focusing, or the like, allow for rapid focusing in contrast with the front lens focusing system but compromise with difficulty in well compensating for the varied aberration, and accordingly, it is an object of the present invention to provide a zoom lens that has merely a single group of lens pieces among other groups utilized for inner-focusing or rear-focusing to well compensate for varied spherical aberration at the telephoto end during focusing.